Uniformity in the parts of any machine or device means that it can be assembled easily or broken items can be replaced quickly.

Many devices are hand-built by craftsmen to extremely high standards, but this brings its own problems. If a designed component doesn’t fit into a machine, it can be carefully worked until it does; in itself, this is no bad thing, but the altered piece may not now fit into an apparently identical mechanism.

By making identical parts within a fine tolerance, identical machines can be built from any mixture of those parts. Work can be divided up amongst craftsmen, and broken parts can be replaced easily, even by a user! This is particularly useful for the military, as muskets and other equipment can be quickly repaired in the field rather than being thrown away or expensively rebuilt.

This advance became widely known about thanks to American inventor Eli Whitney (1765-1825). He built ten muskets in front of a US Congressional committee, took them to pieces, mixed up the parts, and rebuilt the muskets. The act was impressive, but he had cheated by having all his demonstration pieces carefully handcrafted to extremely exacting standards. He was largely copying the ideas of Frenchman Honoré Blanc.

As a late game technology, by the time Interchangeable Parts may be researched, army and navies could be quite numerous, and upkeep costs could be in the thousands or even tens of thousands. Researching this technology makes upkeep costs more manageable, freeing resources for other buildings or units. In a similar vein, while the 1% extra wealth from buildings this technology generates may be proportionally small, by the late game this can be quite a significant increase.